The present invention is related to the field of document image processing, and particularly to methods and apparatus for reducing the scanning artifacts (Moire pattern) in a digital sampling process that are caused by cross interference patterns.
Typical digital document/printing systems include several subsystems: scanner, image manipulation, image storage, image rendering, and printing subsystems. In the input scanner, there are several artifacts that are associated with the digital sampling process. For example, aliasing effects can cause jagged edges on text borders and Moire interference patterns on scanned halftone images. Furthermore, optical lens blurring will affect the sharpness of the image. Such artifacts affect image quality severely as these artifacts are propagated down the digital document copying/printing process.
A number of image processing methods and systems for enhancement of a digitized image have been developed to compensate for the imperfect degraded digital sampling process. (See, for example, U.S. Pat. No. 4,941,190 to Joyce). It is also known, from U.S. Pat. No. 4,194,221, to Stoffel, to provide for automatic reproduction of multimode originals containing continuous tone, halftone and line copy using an image processing method and apparatus for halftone image reproduction.
In the halftone reproduction process, such as that described by Stoffel, there are two kinds of Moire interference patterns. One is the input scanning Moire and the other is the output printing Moire. Since the halftone image is composed of many microdots that are regularly placed in the image, this halftone xe2x80x9cdot structurexe2x80x9d interacts with a regular pitched sampling process in the scanner operation. The resultant scanning Moire patterns are imbedded in the digital image and are carried through the image processing chain, thereby producing undesirable artifacts on the final printed image.
The output printing Moire is different from the input scanning Moire. It is caused by the interference between the halftone dot structure in the image and the output halftone dot structure created in the normal digital halftoning process. This printing interference Moire is more severe than the scanning interference Moire as far as image degradation is concerned. While the printing Moire problem has been addressed elsewhere, the scanning Moire pattern has not been addressed for the halftone reproduction process.
There is a need for an improved method to reduce the cross interference (Moire pattern) in the reproduction of a scanned halftone image.
This and other needs are met by the present invention which provides a method and apparatus for reducing cross-interference in the reproduction of an image, the method comprising the steps of receiving a grey level value for each of a plurality of original pixels of an image, sub-dividing the plurality of original pixels into groups of original pixels, with each group of original pixels defining a boundary, and locating a new pixel within a boundary of each group. Interpolation is performed among the original pixels of each group to produce a grey level value for the new pixel located within that group. The original pixels of each group are replaced with the new pixels to form a modified image.
The above stated needs are also met by a method of reproducing an image, comprising the steps of scanning an original image to produce a digital representation of the original image as a plurality of original pixels regularly located within the original image and each having a grey level value, generating replacement pixels located within boundaries of the original image, performing interpolation of the grey level values of the original pixels and assigning an interpolated grey level value to each of the replacement pixels, and generating a reproduction of the original image using only the replacement pixels and their assigned interpolated grey level values.
In accordance with another aspect of the invention, there is provided in a method of halftone scanning a document having a plurality of pixels, the improvement comprising the steps of generating new pixels from groups of the pixels, randomly locating the new pixels, and applying bilinear interpolation to generate grey level values for the new pixels.
The use of the method of the present invention resamples the image in such a way so as to break up Moire structure so that it is no longer visible. However, the method also preserves important image and edge information.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.